Nadine Mouton
Anna Shimbulu
Failed contraception is a familiar phrase to some women, who have taken the precautionary measures to be on contraceptives, however, ended up being pregnant.
Contraceptives are not 100% effective and the effectiveness differs for each method used. In addition, the expected effectiveness is dependent on the appropriate use of the contraceptive according to the manufacturer’s recommendation.
Another issue that might contribute to contraception failure is concomitant use with other medicines. To minimise the failure of contraception, doctors when prescribing medicines or a pharmacist when dispensing your medicines, may ask questions such as; “what other medicines are you taking?” This question is asked not necessarily out of curiosity, but because some medicines do not work well if taken together.
Most hormonal contraceptives are metabolised by the liver enzymes, and any increase in their clearance, results in a lower concentration than required to have an effect. There are medicines known to induce liver enzymes, thereby causing hormonal contraceptives to be less effective in preventing unintended pregnancy. For this reason, it is important to be well informed of any medicines that may interfere with your contraceptive’s effectiveness.
The medicines that affect the effectiveness of hormonal contraceptives includes the anti-TB medicines, rifampicin and rifabutin, which are highly potent in inducing the liver enzymes. Others include some antiretroviral (ARV) medicines such as efavirenz, nevirapine and ritonavir (another potent inducer); anticonvulsants such as carbamazepine, phenytoin, phenobarbital, topiramate; the antifungal griseofulvin; and the herbal antidepressant, St. John’s wort.
Although most people believe that antibiotics such as amoxicillin and doxycycline reduce the effectiveness of oral contraceptives owing to their alteration of the gut flora, there is lack of evidence to support this interaction. However, when there is diarrhoea and vomiting in the presence or absence of these antibiotics, then the issue of contraceptive inefficacy is a concern. This is because of the possibility that the contraceptive was not sufficiently absorbed from the gut.
The hormonal contraceptives that are affected by the above-mentioned liver enzyme inducers include:
Combined oral contraceptives (e.g Diane-35, Ginette, Yasmin, Yaz, Minulette, Nordette, Ovral)
Progestogen-only oral contraceptives (e.g. Microval
Contraceptive patches (e.g. Evra patches)
Emergency contraceptives (e.g. Norlevo, Escapelle)
Subdermal implants (e.g. Implanon NXT)
Vaginal ring (e.g. NuvaRing)
The knowledge of the concomitant administration of hormonal contraceptives with enzyme inducers helps the healthcare provider to devise ways around possible interactions to ensure that maximal efficacy is maintained. Depending on the contraceptive formulation and duration of treatment with the enzyme inducers, the following are possible solutions:
Continuation of the contraceptives with additional use of condoms
Use of other hormonal contraceptives that are high dose
Consider an alternative method of contraception when potent enzyme inducers are used
Consider an alternative contraceptive not affected by the interacting drug
If you are on a contraceptive and are on any of the medicines mention above or uncertain about possible interactions, call or visit your pharmacist or doctor to make an assessment. In addition, always disclose to your healthcare any medicines that you are taking, even when not asked.
*Nadine Mouton and Anna Shimbulu are medicines information and safety pharmacists. Therapeutics Information and Pharmacovigilance Centre – Namibia Medicines Regulatory Council Secretariat